Rising national rates of whooping cough may signal the reduced efficacy of the acellular pertussis vaccine, government researchers said.

There have been more than 18,000 cases of whooping cough in the U.S. so far in 2012 -- that's twice as many as last year this time -- and the rising rate may signal the reduced efficacy of the acellular pertussis vaccine, government researchers said.

One factor that suggests diminished protection from the vaccine is the increase in pertussis among teens ages 13 to 14 -- those who've received only the acellular pertussis vaccine, not the whole-cell version, Sarah Meyer, an officer with the CDC's Epidemic Intelligence Service, and colleagues reported in the July 20 issue of Morbidity & Mortality Weekly Report.

"The switch we made ... from whole-cell pertussis to acellular pertussis in 1997 may impact how long vaccination lasts," Anne Schuchat, MD, director of the National Center for Immunization and Respiratory Diseases at the CDC said during a Thursday phone briefing with reporters.

All pertussis vaccines in the U.S. are currently made with acellular pertussis. Children receive a five-dose series of the DTaP vaccination beginning at age 2 months, followed by a booster Tdap shot at age 11 or 12.

The vaccination that contained whole-cell pertussis, DTwP, was removed from the market in 1997 because of concerns over a possible -- though unproven -- link to chronic neurologic problems, as well as more severe injection-site reactions and fever, Schuchat said.

Meyer and colleagues also found increases in pertussis rates among 7- to 10-year-olds, further suggesting waning efficacy of the acellular vaccines, since children receive their fifth dose in the DTaP series at age 6.

Schuchat noted that there's been a gradual and sustained increase in pertussis since historic lows in the 1970s, and the last peak year was 2010, with 27,000 reported cases and 27 deaths -- 25 of them infants.

So far this year, the highest rates are in infants under a year old, Schuchat said, with half of cases occurring in infants younger than 3 months who couldn't yet be vaccinated. Nine infant deaths have been reported, the researchers said.

"Their immunity depends on that of the people around them," Schuchat said, urging that all pregnant women and other people who are going to come in contact with infants -- grandparents and healthcare professionals, for instance -- should receive the Tdap booster.

The national trend of increasing rates of pertussis among 13- to 14-year-olds was particularly strong in Washington state, the researchers said, which declared a pertussis epidemic on April 3.

And rates in that age group were high despite high rates of booster vaccination with Tdap, further supporting the premise of waning immunity with acellular vaccines, according to the secretary of the state's department of health, Mary Selecky.

That also means people who refuse to have their children vaccinated probably aren't driving the epidemic, Selecky added.

"That's a bad thing, but we can't blame this whole wave on that phenomenon," Schuchat said.

Selecky said that when the epidemic was declared in April, there were 640 cases of the disease. The toll now stands at more than 3,000.

Rates in Washington were highest in children under a year old, and those ages 10, 13, and 14, Selecky said.

Schuchat said CDC is working with Washington and California to evaluate the effectiveness of the Tdap booster shot and the duration of protection in adolescents who were fully vaccinated with the DTaP series.

Despite questions about the durability of immunity provided by the acellular pertussis vaccines, the researchers wrote that vaccination "continues to be the single most effective strategy to reduce morbidity and mortality caused by pertussis."